Please look here: http://www.openstreetmap.org/?lat=51.52769&lon=-0.03798&zoom=17&layers=M
There is a park, with two lakes, both with islands.
The top one renders properly, the lower one (by “art pavilion”) does not.
I am trying the multipolygon thing, and obviously do not know what I am doing.
I am using JOSM. Please take a look, work out what I have done wrong, and then
please explain to me how I can put it right by using JOSM.
You should move the “natural=water” tag from way to relation. Then the “inner” member of that relation is not drawn as lake.
In the other, there are two relations. 2601555 and 2601556. The …556 is tagged with “natural=water” but it has only two “inner” members. You should add the third one to it. Then delete the …555 relation as it will be obsolete.
" the lower one (by “art pavilion”) does not. "
On this one the “natural=water” tags are on the way (perimeter line of the lake). These tags need to be moved to the multipolygon relation and deleted from the way, as in the uppermost lake multipolygon.
That should sort that problem, I can do that if you wish?
" I have a similar problem going on here: http://www.openstreetmap.org/?lat=51.53 … 8&layers=M "
Firstly do I take that you refer to the two smaller islands? If so them the “natural=land” tag shouldn’t be used, see here http://wiki.openstreetmap.org/wiki/Tag:natural%3Dland
Secondly the lower island has two tags describing the area (landuse=forest and natural=land), I think only one description should be used. May I suggest, after looking at the latest Bing image, that natural=wood, wood=deciduous be used.
For the larger island you could resort to nested multipolygons to make the whole area correct. At the moment the building is not included in any multipolygon. Be warned you can sometimes make more of a mess of things when you start to alter multipolygons. The way I find works is to start from the center and work outwards.
So in that case for the whole lake start by making a multipolygon for the large island, include as an inner the building. Then make a multipolygon of the lake, include the large island multipolygon as an inner, include the two small islands as inners. Job done.
Please see here for further information http://wiki.openstreetmap.org/wiki/Multipolygon and here http://wiki.openstreetmap.org/wiki/Multipolygon_Examples.
Lastly, you’ll like this, the parks in each case describe the whole/ultimate areas, then they should have been the area described by the multipolygon with all inclusive areas within them. That’s going to be even more difficult to rectify now as Mile End Park has two multipolygons, one is the top half of the park the other describes bot top and bottom halves.
There’s quite a lot there to digest but if I can be of further help please ask.
Thank you both for your explanations. I am still confused.
OK, but how?
Again, I do not know how to do this with JOSM. I have the relation toolbox installed, but do not understand how to use it, and I can’t find any documentation.
The wiki page for Multipolygon has lots of markup code, which is readable and understandable, but I don’t see this stuff anywhere in JOSM. I feel as though I am missing a “View - Sourcecode” option in JOSM.
I am tempted to ask one of you just to fix things, but I am trying to understand why and how it is that I went wrong.
In essence, I created two circular ways, the lake and the island. The lake I tagged as natural:water, the island as leisure:park. Then I selected them both and clicked Multi in the relation toolbox. At one time I was warned that the island was drawn the wrong way, and I had to reverse the direction of its way.
I am beginning to think that it would be better for me to stick to roads and buildings in future. Multipolygons are giving me a headache.
It is a bit daunting especially as there’s sometimes more than one way of doing things.
In JOSM download the area to work on. Tip with multipolygons it pays to make sure you download the whole of the multipolygon areas.
Make sure “Properties for selected objects” is enabled (Alt+Shift+P).
Click on the way that denotes the lake, (the line you’ve drawn around it).
You should see the tags natural=water, delete them by highlighting them then click delete
Also you should see the Multipolygon relation under Properties, Member of.
Click or double click on the Multipolygon to bring up the Edit relation dialogue.
In the top half of the editor there’s a list of Keys and Values, in the next spaces downwards (under “type” and “multipolygon” insert Key natural and Value water click OK at the bottom of the editor.
That’s the first job done. Remember if you’re worried and think you’ve messed things up you can always back out. You can save what you’ve done to work on it later, uploading it later when you’re happy with it. Or as I quite often do, chicken out altogether by deleting the data layer without uploading anything, a popup asks you to confirm the delete.
HACY,
Thanks for your links. I had been searching on “JOSM multipolygons” and not “JOSM relations”. Eventually I’ll understand the difference!
BC,
Thanks for the more detailed explanation.
I have made some changes, and am waiting for the tiles to re-render.
Do small islands like these (only a few nodes) have to be defined as multipolygons / relations? What is the advantage? I know that for large areas with many nodes there’s a limit for the number of segments you can draw and connect together, but for something as small as this, does it really matter?
I think you can tag a lake with natural=water and then add an island inside by tagging place=island or place=islet. However in http://wiki.openstreetmap.org/wiki/Island#Islands_in_waterways
“Island within lakes or rivers should be mapped using multipolygon relations…”